Non-conducting low hysteresis rubber treads are used in tires for achieving a lower rolling resistance but have the disadvantages of not discharging the electro static charges generated and accumulated during drive. A common solution to this problem consists in providing a narrow strip of conductive rubber at a widthwise center region of the tire tread. To that end, it is known from U.S. Pat. No. 6,746,227 B2 to form a narrow strip of conductive rubber by conveying the conductive rubber of a lower under tread flow channel in a tire tread die through a passage into a chimney block positioned in the flow channel for the non-conductive tread rubber and through a slot in the chimney block. It results that a narrow strip is formed with its face at the tread surface and in position for discharging static electricity generated by the rolling vehicle. The chimney block is formed on a splice bar positioned in the die in order to splice the two treads of different rubbers.
Details of such a splice bar are illustrated in FIG. 1. It shows indeed a splice bar 1′ with an upper surface 4′ for conveying the non conductive rubber of the upper tread and a lower surface 5′ for conveying the conductive rubber of the under tread. Positioning and conveying blocks 6′ and 7′ are arranged at both ends of the splice bar in contact with the upper and lower surfaces. The upper and lower treads are united when leaving the surfaces of the bar. A chimney block 2′ is arranged at the upper surface 4′, where this chimney block is arranged with a slot 3′ communicating with the lower surface 5′ via a chimney 8′ and a chimney arrival channel 9′.
This arrangement is satisfying for the extrusion of passenger tire treads but not for thicker tire treads like, for example, truck tires. Indeed, the flow of rubber in the slot for forming the narrow strip is too low or irregular in the upper section of the slot. This results in a not fully formed strip, a bad contact with the upper tread and more generally in an unsatisfactory upper tread.
Additionally, the machining of the splice bar is rather complex due to the complexity of the passage as is illustrated in FIGS. 2a, 2b, 2c and 2d. Indeed, a bar after being cast, forged or machined to a trapezoid cross section is machined in five operations requiring four different fastenings. FIG. 2a illustrates the first fastening where the splice bar is positioned so that the lower surface 5′ (as illustrated in FIG. 1) substantially faces upwards. This position is for machining the surfaces of the base side of the splice bar and also for machining the chimney arrival channel in the base side. FIG. 2b illustrates the second fastening for machining. This position corresponds essentially to the position of FIG. 1 the cap side. FIG. 2c illustrates the third fastening for drilling the chimney. The splice bar is positioned with the lower surface 5′ (as illustrated in FIG. 1) approximately face upwards like in FIG. 2a whereas the bar is inclined about its longitudinal axis for bringing the location of the chimney to the vertical. FIG. 2d illustrates the fourth fastening for machining the slot through which the narrow strip of conductive rubber will be extruded. The downstream side of the chimney block faces upwards in this position.